Antifungal activity against Fusarium oxysporum of quinolizidines isolated from three controlled-growth Genisteae plants: structure–activity relationship implications

The Genisteae tribe belongs to the Fabaceae family. The wide occurrence of secondary metabolites, explicitly highlighting the quinolizidine alkaloids (QAs), characterizes this tribe. In the present study, twenty QAs (1–20), including lupanine (1–7), sparteine (8–10), lupanine (11), cytisine and tetrahydrocytisine (12–17), and matrine (18–20)-type QAs were extracted and isolated from leaves of three species (i.e., Lupinus polyphyllus ('rusell' hybrid), Lupinus mutabilis, and Genista monspessulana) belonging to the Genisteae tribe. These plant sources were propagated under greenhouse conditions. The isolated compounds were elucidated by analyzing their spectroscopical data (MS, NMR). The antifungal effect on the mycelial growth of Fusarium oxysporum (Fox) of each isolated QA was then evaluated through the amended medium assay. The best antifungal activity was found to be for compounds 8 (IC50 = 16.5 µM), 9 (IC50 = 7.2 µM), 12 (IC50 = 11.3 µM), and 18 (IC50 = 12.3 µM). The inhibitory data suggest that some QAs could efficiently inhibit Fox mycelium growth depending on particular structural requirements deduced from structure–activity relationship scrutinies. The identified quinolizidine-related moieties can be involved in lead structures to develop further antifungal bioactives against Fox. Supplementary Information The online version contains supplementary material available at 10.1007/s13659-023-00373-4.


Compounds 1-7: Lupanine-type
Compounds 1-7 were isolated from the leaves of L. polyphyllus 'rusell' and L. mutabilis. Each lupanine-type QA was structurally characterized by the presence of a bridged tetracycle with a lactam group in the A ring. Compounds 2 and 3 were structurally related to compound 1, but these differ by the presence of a double bond in D-ring (in 2) and in A-ring (in 3). GC-MS and HRMS analyses of 2 and 3 showed the same molecular formula C15H22N2O, indicating an isomeric relationship. To establish the difference between isomers 2 and 3, 1 H NMR was then used.

Compounds 8-10: Sparteine-type
Compound 8-10 were isolated from the leaves of L. mutabilis and G. momspessulana. Compounds 8-10 are related to those of the lupanine type because they are bridged tetracycles; however, they differ structurally due to the absence of the carbonyl group in compound 8. In the case of compounds 9 and 10 they contain carbonyl group in positions 4 (ring A) and 10 (ring B) respectively.

Compound 8: (-)-sparteine
Compound 8 ([ ] 20 = -20, MeOH, c 0.01) was a brown oil (103.7 mg), positive for Dragendorff's reagent, and soluble in chloroform, methanol, and water. The analysis by GC-MS afforded an m/z = 234 that corresponds to the molecular formula C15H26N2. This information was confirmed by HRESIMS, and a [M+H] + = 235.2184 (calcd 235.2174) was obtained. The structure of compound 8 was confirmed by 1 H and 13 C NMR, whose signals were compared with the literature and agreed with the data reported for (-)-sparteine, isolated from L. albus [1,9].

Compound 11: Lupinine-type
Compound 11 was isolated from the leaves of L. polyphyllus polyphyllus 'rusell'. Compound 11, a lupininetype QA, is characterized by an azabicyclic moiety, whose main building block is a quinolizidine.  calcd 170.1544) was obtained. The structure of compound 11 was confirmed by 1 H and 13 C NMR, whose signals were compared with the literature and agreed with the data reported for (-)-lupinine, isolated from L. luteus [12,13].

Compounds 12-15: Cytisine-type
Compounds 12-15 were isolated from the leaves of G. monspessulana. Each cytisine-type QA was structurally characterized by the presence of a bridged tricycle with a 2-pyridone in the A ring. Compounds 13 and 14 were structurally related to compound 12, but these differ by the presence of a substitution in the nitrogen at position 12 (methyl and formyl). In case of compound 15 it is related to compound 1 by being a bridged tetracycle and it is also related to compound 12 by the presence of the 2-pyridone in the A-ring.

Compounds 16-17: Tetrahydrocytisine type
Compounds 16-17 were isolated from the leaves of L. polyphyllus 'rusell'. Each tetrahydrocytisine-type QA was structurally characterized by the presence of a bridged tricycle with a lactam group in the A ring. Compounds 16 and 17 were structurally related to compound 12, for being tricyclic, but these differ by the absence of the 2-pyridone from ring A.